This invention relates to an active matrix display device using two-terminalnon-linear switching devices, and in particular a display device comprising sets of row and column address conductors, a row and column array of electro-optic display elements operable to produce a display, each of which is connected in series with a two terminal non-linear switching device between a row conductor and a column conductor, and a drive circuit connected to the sets of row and column address conductors for applying selection signals to the row address conductors to select the rows of display elements and data signals to the column address conductors to drive the selected display elements to produce a required display effect. The invention is concerned also with methods of driving such display devices.
The display device may be a liquid crystal display device used to display alpha-numeric or video information. The two terminal non-linear switching devices commonly used in such matrix display devices comprise thin film diode devices, MIMs, diode rings or back to back diodes which are bidirectional and largely symmetrical. The capacitive display elements in these devices are addressed by sequentially applying a selection voltage signal to each one of the set of row address conductors in turn in a respective row address period and applying in synchronised relationship data signals to the other set as appropriate to charge the display elements to a level providing the desired display condition, which following the row address period is subsequently held to maintain the display condition until they are again selected in a following field period. Conventionally, the data signals comprise amplitude modulated (analogue) voltage pulse signals of substantially identical and constant duration, related to the duration of the row address period, and whose amplitudes are varied to determine the display element voltage and produce the display effect required.
Display devices of the above kind and methods of driving such are described in U.S. Pat. No. 5,159,325 and GB-A-2129182. The method described in GB-A-2129182 entails the application to each row address conductor of a four level row drive waveform comprising a selection voltage level for a row selection interval of fixed duration followed by a second, hold, voltage level of less value but of the same polarity as the selection level which serves to hold the switching devices in the row off and which is maintained for at least a major portion of the time which elapses until the row conductor is next addressed so that the display elements are kept substantially at the level to which they were driven for that period. In successive field periods, the polarity of the selection and hold levels is inverted, thus making a four level signal waveform for each row conductor.
The method described in U.S. Pat. No. 5,159,325 employs a five level row scanning drive waveform which includes a reset voltage signal in addition to the selection signals and non-selection (hold) levels. The selection and hold levels are changed for successive fields and, together with the reset voltage signal, which may be regarded as an additional selection signal, form a five level signal waveform. Before presenting a selection signal, which together with the data signals provides the display elements of a row with a voltage of a certain sign, the display elements are charged through their non-linear switching devices to an auxiliary voltage level of the same sign and which lies at or beyond the range of voltage levels (Vth to Vsat) used for display purposes. This drive scheme helps to compensate for the effects of differences in the operating characteristics of the switching devices of the display device. Ideally, these devices should demonstrate substantially identical threshold and I-V characteristics so that the same drive voltages applied to any display element in the array produce substantially identical visual results. Differences in the thresholds, or turn-on points, of the non-linear switching devices can appear directly across the electro-optical material producing different display effects from display elements addressed with the same drive voltages.
Problems can arise if the operational characteristics of the switching devices drift over a period of time through ageing effects causing changes in the threshold levels. Because the voltage appearing across the electro-optic material depends on the on-current of the non-linear device, then if the on-current changes during the life of the display device the voltage across the electro-optic material also changes, which leads to inferior display quality and image storage problems. For switching devices such as thin film diode devices it has been found that this is ageing is due to current stressing effects. In EP-A-0699332 a modification to the form of the selection signals is proposed for reducing the extent of ageing effects. The form of the selection signals is tailored so that the peak current flowing through a switching device upon addressing, and thus the extent of ageing, is reduced. The difference in ageing between switching devices associated with display elements continually driven to different levels is also reduced. This is achieved by arranging that the selection voltage signals applied to the row conductors comprises a shaped voltage pulse signal whose magnitude increases gradually in a controlled fashion to a maximum amplitude during the row address period rather than the usual generally rectangular shape whose leading edge has a rapid and uncontrolled rise time which results in a high peak of current flowing through the device at the start of the selection address period. Through this shaping of the selection signals, the waveform of the current flowing through a switching device has a significantly reduced peak level.
In all these display devices the data signals applied to the display elements via the column conductors comprise amplitude modulated voltage signals whose level, together with the level of the selection signal, determines the voltage level of the display element, and thus its grey scale level, at the end of row address period.
Proposals have been made to drive an LC display device using two terminal non-linear switching devices by means of a pulse width modulation (PWM) drive scheme. This kind of drive scheme can offer attractions in certain types of display applications, particularly datagraphic, as purely digital, and hence for example lower power and less expensive, drive circuit ICs can be used. However, these proposals have generally proved unsatisfactory. GB-A-2186414 describes a PWM drive scheme but this involves a multiplex type drive technique rather than a true active matrix addressing technique. Unlike the above described row drive waveforms which include hold levels between successive selection signals that alternate in polarity in successive, positive and negative, fields the voltage present on the row conductors in the interval between selection signals is the same in both positive and negative fields. This means that the voltage on a display element capacitance decays away during the interval and the main contribution to the rms voltage across the LC display element is a voltage spike which occurs during the row address (selection) period only. The consequence of this is that the response speed of the LC material must be several field periods long in order to avoid flickering effects and this leads to a very slow response to changes in image content. Furthermore, the width (duration) of the selection signal is much more critical and a short selection signal duration can not be achieved without excessive drive voltage levels. In EP-A-0619572 a PWM drive scheme for a MIM LC display device is described in which a four level row drive waveform, having selection signals and hold levels that alternate in polarity in successive positive and negative fields, and similar to that described in GB-A-2129182, is used and in which the data signals determining grey-scale comprise pulse width modulated signals. However, it has been found that the range of grey-scales possible with the drive scheme described is severely limited so that the display device is not suitable for many display applications.